Two countries who had the biggest turn around in relations over night?

Are there any countries that have been close friends then become enemies seemingly overnight? What about vice versa? The elephant in the room is ofc Canada and The USA. In my opinion they were like brothers and now Canada has stickers on products telling shoppers which ones were made in America so they can avoid buying them. Are there any other country relationships who have big turnarounds like this?

Why is the north eastern part of China (specifically the coastal region) hotter than the surroundings?

Shoutout to Ngerulmud, Palau, the smallest capital city in the world

Population: 0 Land area: 0.45 km^(2)